Interim Standard IS-2000 has been adopted by the Telecommunications Industry Association for implementing cdma2000® in a cellular system. In a cdma2000 communication system, a mobile station (MS) communicates with any one or more of a plurality of base stations (BSs) dispersed in a geographic region. Each BS continuously transmits a pilot channel signal having the same spreading code but with a different code phase offset. Phase offset allows the pilot signals to be distinguished from one another, which in turn allows the BSs to be distinguished. Hereinafter, a pilot signal of a BS will be simply referred to as a pilot. The MS monitors the pilots and measures the received energy of the pilots.
The MS communicates with a BS providing wireless communication services to the MS via a forward link and a reverse link. The forward link typically includes one or more forward traffic channels, one or more forward control channels, and a paging channel. The reverse link typically includes one or more reverse traffic channels, one or more reverse control channels, and an access channel. During a call, the MS must constantly monitor and maintain four sets of pilots. The four sets of pilots are collectively referred to as the Pilot Set and include an Active Set, a Candidate Set, a Neighbor Set, and a Remaining Set. The Active Set comprises pilots associated with a forward traffic channel assigned to the MS. When the MS is in an idle mode, the Active Set comprises pilots associated with a paging channel or a forward control channel monitored by the MS. The Candidate Set comprises pilots that are not currently in the Active Set but have been received by the MS with sufficient strength to indicate that an associated forward traffic channel could be successfully demodulated. The Neighbor Set comprises pilots that are each transmitted from a BS to the MS and are possible candidates for handoff. The Remaining Set comprises all possible pilots in the current system on the current CDMA frequency assignment, excluding the pilots in the Neighbor Set, the Candidate Set, and the Active Set.
Typically, a BS provides communications services to MSs located in a coverage area serviced by the BS. When the MS is serviced by a first BS, the MS constantly searches pilot channels of neighboring BSs for a pilot that is sufficiently stronger than a threshold value. The MS signals the determination of a pilot that is sufficiently stronger than the threshold value to the first, serving BTS using a Pilot Strength Measurement Message. As the MS moves from a first coverage area serviced by a first BS to a second coverage area serviced by a second BS, the communication system promotes certain pilots from the Candidate Set to the Active Set and from the Neighbor Set to the Candidate Set. The serving BS notifies the MS of the promotions. Then, when the MS commences communication with a new BS that has been added to the Active Set before terminating communications with an old BS, a “soft handoff” has occurred. For the reverse link, typically each BS in the Active Set independently demodulates and decodes each frame or packet received from the MS. It is then up to a switching center to arbitrate between the each BS's decoded frames.
When an MS is operating in a high velocity environment, such as when the MS is located a high speed aircraft such as a commercial airplane, a conventional operation and handoff of the MS is fraught with problems. Typically, MSs include a mechanism for correcting for Doppler shifts in received signals resulting from the MS traveling at speeds of less than 120 kilometers per hour. However, when an MS is traveling at a speed in excess of 120 kilometers per hour, the MS is unable to fully compensate for the resulting Doppler shift in a received signal, resulting in an inability of the MS to properly demodulate the received signal. Furthermore, when an MS is traveling at an excessively a high rate of speed, new handoffs may be indicated before current handoffs may be completed, resulting in system inefficiencies and potentially resulting in dropped calls. Furthermore, pilots and associated BSs may be transferred in and out of the Neighbor Set so frequently that the Neighbor Set becomes somewhat useless.
Therefore, there exists a need for a method and apparatus for performing handoffs and Doppler compensation with respect to an MS traveling at a high velocity.